Unless you're going to create an entirely new continuity, I honestly think that the most organic way to do an X-Force film is to use DoFP to establish that the 2023 dystopian future from that movie still exists - albeit not as dangerous thanks to whatever was done in the past - and have said future be the setting for the X-Force movie. That way, you've got both a loose connection to the X-Franchise as well as the creative freedom to pretty much tell whatever stories you want.

Doing this would mean no cross-polienization of the franchises unless you made it clear that the X-Force timeline was different from the timeline of the 'X-Franchise', but I don't think doing crossovers would be the best option anyway.